Bentonite, also known as amargosite or swell soil, is clay rock mostly consisting of montmorillonite. The chemical formula of montmorillonite is 4SiO2.Al2O3.H2O, and the crystal structure of montmorillonite is a 2:1 phyllosilicate which is formed in such a manner that two silicon-oxygen tetrahedral sheets sandwich one aluminum-oxygen octahedral sheet. Bentonite has a series of physicochemical properties such as water absorbability, swelling capacity, dispersion and suspension property, cohesiveness, the absorbability of organics, catalytic activity, thixotropy, plasticity, lubricity and cation exchange capacity and the like, as well as good thermostability and chemical stability.
Bentonite has numerous excellent performances due to its unique mineral structure and chemical property of crystallization. Accordingly, bentonite, especially modified bentonite, is widely used in various industries, such as, as a mud thickener, an emulsion stabilizer, a plasticizer, an adsorbent or a lubricant in the oil drilling industry, casting and metallurgical industry, coating and ink industry, farming and livestock husbandry and textile industry, paper industry, daily chemical industry and food processing, and petrochemical industry, as a clean agent or an adsorbent in environment protection, and as clay raw materials of low iron siliceous in producing bentonite flashings, bentonite daub and bentonite sealing rod and the like.
However, although bentonite has excellent water swelling capacity, dispersion and suspension property, water-holding capacity, lubricity, stability, etc., either non-modified bentonite nor modified bentonite in the prior art is fit for being blended into cement or concrete as waterproof additives, waterproof and impervious additives, or additives capable of resisting permeation and salt corrosion. The reason for this lies in that, montmorillonite, the main mineral of bentonite, is a type of clay mineral, which still has the properties of clay minerals. In the complex liquid medium environment of ionic-type strong alkaline cement concrete, under the combined action of hydrated calcium silicate gel, sulfoaluminate, aluminoferriate, the crystal mineral of calcium hydroxide, and complex ionic-exchanges in the course of cement hydration, a fusion zone or strengthening transition zone could not be effectively formed between bentonite or modified bentonite and hydrated minerals of cement. Besides, the bentonite is either subject to untimely full-expansion, which gives rise to its separation from hydrated minerals of cement and formation of dried-up channels, or to damage or suppression of the original excellent performances. In addition, the cementing property of the hydrated minerals of cement is weakened simultaneously by clay minerals.
Cement, as the cementing material of concrete, is the most widely used hydraulic cementing material. After hardening, a large number of microchannels and dried-up, heat-expansion microcracks are generated. These microchannels and microcracks provide paths and create conditions for permeation and salt corrosion, which result in permeation and salt corrosion of cement, concrete products, or concrete constructions. Especially in the engineering constructions such as dams, tunnels, culverts, ditches, pools, towers, dikes, wharf, etc., the problems of resisting permeation and salt corrosion become the hardest engineering technical problem.
Currently, the technical methods for solving the problems of leakage and salt corrosion existing in cement and concrete works can be summed up in four categories: setting up one or more isolating layers for resisting permeation; blending an expanding agent such as calcium sulfate to make the work more dense; blending a large quantity of expensive high polymer material to block pores and cracks; using special cement; or employing the combination of the above categories of methods. These methods result in high costs of constructions and maintenances in different degrees.
In addition, when cement is mixed with dinas to produce concrete for construction, during the process of hydration and hardening thereof, the water on the surface of the concrete evaporates and runs off easily. In particular, for lowering production cost, current cement manufacturers add a large quantity of industrial waste residue or non-plastic material such as limestone, raw gangue, or black shale into cement. Rather low surface energy and poor association capacity with water with respect to the waste residue or filter particles in the cement, would lead to the rapid water loss and thus drying of the surface or almost the entirety of the concrete. Furthermore, the particles of silicate minerals in cement cannot be hydrated to be calcium silicate gel and crystalline, and the waste residue particles with potential activity in cement also lose the alkaline water and cannot be hydrated, thus resulting in low strength on the surface of the concrete, serious sugaring, and even spongy surface.
At present, the methods for solving sugaring of cement concrete can be roughly classified into three categories: covering the surface with films or petates and spraying water; in the course of cement production or cement mixing, adding a large amount of water-soluble macromolecular water-retaining material, or spraying water at regular time for humidification simultaneously; spraying a water retention and isolating agent on the surface. Because of economic reasons or the incompatibility of organic and inorganic substances, the above mentioned methods can be performed merely in important projects.
For the above reasons, at home and abroad there still lacks a modifier which enables bentonite to play the role of waterproofing and impermeability, and a texturizer which can adjust and improve the performance of bentonite in the medium environment of cement concrete, so that there is a shortage of a bentonite modifier which can be used in waterproofing, resisting permeation and salt corrosion, avoiding sugaring of cement concrete as well as bentonite modified with said bentonite modifier.
In addition, during the process of cement production, generally, due to the blasting action, either “kiln dust”, or “kiln dust” and recycled ash” is generated. “Kiln dust”, as the name implies, is the fly ash collected by a rear kiln cyclone and a dust-precipitator and is taken along with flue gas from the interior of the rotary kiln where the clinker is prepared by calcining.
Currently, kiln dust released by cement plants can be classified into two categories. One is the kiln dust released by the rotary kilns of the ordinary dry process, wet process or semi-dry process, comprising fly ash from the interior of the rotary kiln that leaks or escapes from the pre-heater kiln (without a decomposition furnace) system. The other is the fly ash (kiln dust) with a high content of chlorine-alkali from the interior of the rotary kiln, released by the bypass system provided at the rear kiln chamber or the uptake of the rear kiln chamber in the new dry process cement rotary kiln (with a decomposition furnace). The fly ash has different properties from the “recycled ash” released by the pre-heater system of new dry process kilns. The properties of kiln dust are between those of clinker and raw materials. The high temperature kiln dust released by the feed-end chamber bypass system of the new dry process cement kiln is generally added into a cement mill as a cement admixture, and other kiln dust of rotary kiln generally returns to the interior of the kiln with raw materials to be utilized or added into a cement mill as a cement admixture.
“Recycled ash” means the superfine particles, which is released during production of new dry process rotary kiln with waste gas from a decomposing furnace and a five-stage pre-heater. The new dry process rotary kiln provided with the by-pass system would generate numerous “kiln dust” from the interior of the rotary kiln and the kiln end chamber. Generally, the dust efficiency of the first-stage cyclone of the pre-heater in a kiln system is designed to be in the range from 95% to 97%, and the amount of superfine powder released along with the high temperature waste gas higher than 300° C. at the outlet of the first-stage cyclone is often more than 5% of the adding amount of raw materials. Such superfine powder is recycled via a humidifier tower and a dust-precipitator and referred as the recycled ash. Currently, when the mills and the kiln are opened synchronously, the recycled ash, together with the recycled ash of raw materials collected by the raw material mill, is continuously added into the raw material powder ground by the raw material mill, so as to be conveyed into the raw material homogenizing silo, or be conveyed into the raw material silo directly, or when the raw materials mill stops milling or the system raw materials silo malfunctions, be conveyed into the kiln elevator directly. Compared with ordinary raw materials having a controlled composition ratio, the recycled ash of the kiln system has a higher saturation ratio, lower silica ratio, and higher contents of aluminium and iron, as well as typically higher content of harmful elements. In other words, there is a big composition difference between the recycled ash and ordinary raw materials, which significantly affects the kiln system. Especially, when the raw material mill stops and the kiln is open, the accumulation of the recycled ash in the raw material silo would results in abnormal fluctuations of the raw material compositions, which has a strong impact on the kiln conditions and the yield and quality thereof. Therefore, when the raw material mill stops, there is no other choice than directly feeding the recycled ash into the kiln system, but the composition problem of the recycled ash will still seriously affect the kiln conditions and the product quality thereof. Besides, as the recycled ash enters the raw material homogenizing silo or directly enters the kiln, the elementary compositions which are bad for the production in the dry process rotary kiln such as chlorine, alkali and sulfur and the like contained in the recycled ash would circulate and gather, which may cause substantial harm to kiln conditions and product quality. Therefore, some enterprises have to set up a by-pass system (generating kiln dust) to reduce the damage caused by the circulation and enrichment of chlorine, alkali and sulfur, and also a small part of enterprises have to discard part of recycled ash.
On the other hand, in order to reduce production cost, in the current cement production, 20% to 50% industrial waste residue by weight of the cement as blending materials need to be added, depending on the types and strength grades of the cement, followed by being ground together, or after grinding and powder selecting a large quantity of waste residue is added into clinker powder or cement powder, to produce finished cement. Currently, the quantity demand for blending materials or slag powder is high, and therefore it is difficult to meet the demand of cement production. To reduce the production cost, cement manufacturing enterprises have to use a large quantity of limestone, even raw gangue, black shale, clay minerals as blending materials. These blending materials that seriously degrade the performance of cement concrete have caused a large number of bad engineering projects.